If you read the T&C, or even just what they say online, you'll see that on Lufthansa it isn't the amount of baggage you have but the TOTAL weight of your checked baggage. When you get "Gold Status" you are allowed an extra 20kg in your allowance which means you COULD/MIGHT be able to check in an extra bag depending on which class you are travelling in and on which route. Economy gives you a 20kg limit, First gives you 40kg where they use the "Weight Concept" on some routes, where they use the "Piece Concept" it is one case max 23kg in Economy and 2 cases max 32 kg in First/Business. You'll also see the same policy as the "Piece Concept" on the United website.
So, in other words, you've misread the terms and gone over your personal weight limit and have been charged accordingly